If the store has their merchandize back, you have no moral obligation to enrich them. Not every attorney agrees, but most attorneys here on AVVO would say you shoud ignore the civil demand letter. Any action under the civil demand letter cannot result in a criminal conviction. If they sue you in court for money, you can pay or consult an attorney - anything but ignore the suit. The letter is not a lawsuit.
We do not have a client/attorney relationship until you make an appointment, we discuss your case face to face, I accept a retainer, and we explictly agree to enter into representation.